Process of manufacturing insulated heater wire and article



Dec. 15, 1964 w. H. KINGSLEY ETAL 3,161,540

PROCESS OF MANUFACTURING INSULATED HEATER WIRE AND ARTICLE Filed June20, 1962 INVENTORS ne ,4. K/hgs/ey BY /as 5. Monroe ATTORNEY UnitedStates Patent 3,161,540 PRGCESS OF MANUFACTURING INSULATED HEATER WIREAND ARTICLE Wayne H. Kingsley and Douglas 5. Monroe, Emporium,

Pa., assignors to Sylvania Electric Products Inc., a corporation ofDelaware Filed June 20, 1962, Ser. No. 203,962 6 Claims. (Cl. 117-4217)This invention relates generally to the manufacture of an insulated wireand more particularly to a process for forming an insulated wire whichmay be used as a heater for an electron discharge device.

In the manufacture of heaters for electron discharge devices, theprocess of depositing a binder suspension containing aluminum oxide upona refractory wire to provide a conductor having an insulating layer ofaluminum oxide has been extensively used for a number of years. In thisprocess, it has been found that refractory wires such as tungsten have amultitude of surface contaminants and oxides which must be removedbefore adequate adherence of the insulating layer can be obtained.Accordingly, the wire is heated in a reducing atmosphere prior to theaddition of the insulating layer to substantially remove the surfacecontaminants and oxides.

Thereafter, a layer or multiple layers of the binder suspensioncontaining the aluminum oxide are deposited on the cleaned wire.Further, during this period of suspension deposition or shortlythereafter, the coated wire is subjected to heat in order to remove thevolatile constituents of the binder and cause the aluminum oxide toadhere to the refractory wire. During this coating process, it has beenfound that the heat necessary for the removal of the volatile binderconstituents and to cause adherence of the aluminum oxide to the wire iswithin the range of the heat at which oxides are formed on therefractory wire. The deleterious presence of oxides on the wire duringthe manufacturing process is difiicult to detect and control and theirpresence results in excessive wire loss and increased expense due topoor adherence of the insulating layer. Additionally, it is believedthat oxides between the wire core and the insulating layer provide thebasis for electrical leakage between the heater and the cathode of anelectron tube, increased hum when the heater is activated by an A.C.voltage, and eventual burn out or failure of the heater during sustainedoperation.

Therefore, it is an object of this invention to improve the adherencebetween the refractory wire and a layer of insulation provided thereon.

Another object of this invention is to improve the permissible range ofoperating heat employed during the process of depositing a layer ofinsulating material on a refractory wire.

A further object of this invention is to reduce the leakage currentlevel between the heater and the cathode in an indirectly heatedelectron discharge device.

A still further object of this invention is to enhance the lifecharacteristics of an electron discharge device.

The foregoing objects are achieved in one aspect of this invention bythe provision of a noble metal barrier film intermediate the refractorywire and the layer of insulating material to inhibit oxide formation onthe wire as well as any chemical reaction between the binderconstituents of the deposited layer and any oxides formed on the wireduring the insulator forming operation.

It is to be understood that any refractory metal wire which may be usedas a heater in an electron discharge device is applicable to thismanufacturing operation although tungsten, molybdenum iand alloysthereof are preferable. Further, the noble metal barrier film may begold, platinum, rhenium or other metals characterized by a high workfunction value, high resistance to oxidation 3,161,540 Patented Dec. 15,1964 and otherwise relatively inert, and a comparatively highvaporization temperature.

Additionally, the binder suspension wherein the insulating material iscontained may be a volatile binder such as the nitrates of aluminum,magnesium, beryllium and like metals or a volatile organic binder suchas ethyl cellulose, methyl methacrylate, or polyvinyl alcohol employingsolvents such as water, alcohol, acetone, methyl acetate, toluene, orsuch volatile solvents as are appropriate for the binder selected.Moreover, the insulating material may be the oxides, silicates, orcombinations thereof of such metals as aluminum, beryllium, titanium andother For a better understanding of the invention, together with otherand further objects, reference is made to the following disclosure andappended claims in connection with the accompanying drawings in which:

FIG. 1 illustrates an insulated wire formed for use as a heater in anelectron discharge device; and

FIG. 2 is a sectional view of an insulated wire having a barrier filmthereon.

Referring to the drawings, FIGURE 1 is a folded heater having aninsulated layer 7 such as aluminum oxide aifixed to a refractory wire 3such as tungsten. In FIG- URE 2 is shown an insulated wire with a noblemetal film 5 such as gold disposed intermediate the insulating layer 7and refractory wire 3.

The insulated wire may be made into any conventional form such as thefolded heater of FIGURE 2 and inserted within a cathode of an electrontube. It has been found that the activation and operation of an electrontube requires a heater temperature of more than 1000 C. Thus, when theinsulated wire having a barrier layer 5 is used as a heater in anelectron tube, the temperature required for the correct activation andprocessing thereof vaporizes the gold film 5 from the tungsten Wire 3.The gold appears to disperse within the aluminum oxide coating 7 and todeposit upon the inner surface of the cathode. Accordingly, the highwork function value and inert properties of the gold are utilized toreduce the potential current leakage properties of the aluminum oxidelayer 7 and the field emission of the conventional nickel cathode innersurface. Hence, leakage currents between these elements, A.C. hum, andheater embrittlement are substantially reduced during operation of theelectron tube.

In the process of forming the insulated wire, the gold film 5 may bevapor deposited, electroplated, or otherwise affixed to the tungstenwire 3. Thereafter, the film covered wire may be subjected to spray,immersion cataphoretic, drag or other depositing processes wherein avolatile binder suspension of the types described above containing theappropriate insulating material is deposited on the wire. Subsequently,the wire is heated to volatilize the binder constituents, harden theinsulating layer, and adhere the insulating layer to the gold film. Theinsulated wire may then be formed into a heater, inserted into thecathode of an electron tube, and heated to activate the cathode of thetube whereupon the gold film is vaporized from the core wire.

Specifically, the tungsten wire 3 has deposited thereon the noble metalor gold film 5 within a range of 0.2% to 4.0% by weight of the corematerial. A film of this thickness and continuity has been foundsuflicient to prevent oxidation of the tungsten wire but insufiicient toadversely affect the operation or appearance of surrounding elements inan electron tube. This film covered wire then has deposited thereon atacky aluminum nitrate binder of the type described in United StatesPatent No. 2,179,- 453 having an oxide of aluminum suspended therein.This layer coated wire is then heated in air between 600 C. and 800 C.in order to volatilize the binder constitu- 3 ents and adhere thealuminum oxide layer to the gold film. The tungsten wire 3 with adheringgold film 5 and aluminum oxide layer 7 may then be formed into a heateras shown in FIGURE I, inserted into an indirectly heated cathode andraised to a temperature above 1000 C. whereupon the gold film isvaporized from the tungsten wire.

It has been found that the binder volatilization and insulator bakingtemperature of from 600 C. to 800 C. provides optimum adherence of theinsulating materials recited above to the enumerated refractory coreheater wires. Also, the conventional potentially electron emissivecathode materials like the earth metal carbonates generally require anactivation temperature above 1000 C. in order to reduce to the oxidesand metals necessary for the promotion of electron emission.Accordingly, the barrier noble metals, with vaporization temperaturesranging from 850 C. to 2300" C. will remain on the core wire until finaltube processing.

Thus it can be seen that the gold film 5 prevents oxidation of thetungsten wire core 3 during the initial stages of the electron tubefabrication process when oxidation could otherwise readily occur. In thefinal stages of tube fabrication during evacuation and cathodeactivation the gold is vaporized in order to render the insulatingaluminum oxide heater coating relatively inert and further to lower thefield emission characteristics of the inside surface of the nickelcathode sleeve which surrounds the heater. Accordingly, heater cathodeleakage, A.C. hum, and heater embrittlement and failure are markedlyreduced in the operating tube.

While there has been shown and described what are at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:

1. A process for forming an insulated wire adapted for use in indirectlyheated electron tubes comprising the steps of depositing a film of anoble metal on a refractory wire, depositing a coating of volatilebinder and aluminum oxide on said film, heating the coated wire abovethe volatilization temperature of the binder and below the vaporizationtemperature of the noble metal to cause adherence of the aluminum oxideto the film, and subsequently heating the insulated wire to atemperature above the vaporization temperature of said noble metal.

2. A process for forming an insulated wire adapted for use in indirectlyheated electron tubes comprising the steps of depositing a film of goldon a tungsten wire, depositing a coating of volatile binder and aluminumoxide on said film, heating the coated wire above the volatilizationtemperature of the binder and below the vaporization temperature of thegold to cause adherence of the aluminum oxide to the gold film, andsubsequently heating the insulated wire to a temperature above thevaporization temperature of said gold.

3. A process for forming an insulated wire adapted for use in indirectlyheated electron tubes comprising the steps of depositing upon a tungstenwire a gold film having a range of 0.2% to 4.0% by weight of said wire,depositing a coating of volatile binder and aluminum oxide on said film,heating the coated wire above the volatilization temperature of thebinder and below the vaporization temperature of the gold to causeadherence of the aluminum oxide to the gold film, and subsequentlyheating the insulated wire to a temperature above the vaporizationtemperature of said gold.

4. An insulated wire adapted for use in indirectly heated electron tubescomprising a core of refractory wire, a layer of aluminum oxide and avolatile binder, and a barrier film of noble metal intermediate saidwire and said layer.

5. An insulated wire adapted for use in indirectly heated electron tubescomprising a core of refractory wire, a layer of aluminum oxide and avolatile binder, and a barrier film of noble metal with a range of 0.2to 4.0% by weight of the refractory wire intermediate said wire and saidlayer.

6. An insulated wire adapted for use in indirectly heated electron tubescomprising a core of tungsten wire, a layer of aluminum oxide, and abarrier film of gold with a range of 0.2% to 4.0% by weight of thetungsten wire intermediate said wire and said layer.

References Cited in the file of this patent UNITED STATES PATENTS835,938 Cazin Nov. 13, 1906

1. A PROCESS FOR FORMING AN INSULATED WIRE ADAPTED FOR USE IN INDIRECTLYHEATED ELECTRON TUBES COMPRISING THE STEPS OF DEPOSITING A FILM OF ANOBLE METAL ON A REFRACTORY WIRE, DEPOSITING A COATING OF VOLATILEBINDER AND ALUMINUM OXIDE ON SAID FILM HEATING THE COATED WIRE ABOVE THEVOLATILIZATION TEMPERATURE OF THE BINDER AND BELOW THE VAPORI-